Looks like they won't send you to Penny's Bay after all:
https://www.coronavirus.gov.hk/eng/d...returnees.html
If, due to delay of my flight, I arrive in Hong Kong one day behind the schedule, do I need to bear the room cost for the extra night?
The quarantine period should be counted from the actual date of arrival in Hong Kong. Therefore, your compulsory quarantine period will automatically be pushed back by a day, and you have to bear any additional costs on room booking.